1. Field of the Invention
The present invention relates to an alternating current (AC) driven light emitting device.
2. Description of the Related Art
Semiconductor light emitting diodes (LEDs) have advantages as light sources in terms of output, efficiency, and reliability. Research into the development of semiconductor LEDs that are able to replace the backlights of lighting apparatuses or display devices as high-output and high-efficiency light sources have been actively conducted.
In general, LEDs are driven at a low DC voltage. Therefore, an additional circuit (e.g., an AC/DC converter) that supplies a low DC output voltage is required to drive a light emitting diode at a normal voltage (e.g., AC 220V). However, the introduction of the additional circuit may not only complicate the configuration of an LED module, but also reduce the efficiency and reliability thereof during a process of converting supply power. Further, an additional component in addition to a light source increases manufacturing costs and product size, and electro-magnetic interference (EMI) characteristics are deteriorated due to periodic components during a switching-mode operation.
In order to solve these problems, various types of LED driving circuits that can be driven at an AC voltage without using an additional converter have been proposed. However, due to the diode characteristics of the LED, it is difficult to achieve bidirectional AC driving with the use of only the LED. A Zener diode may protect the LED, but it is inefficient in terms of size, capacity and cost. Unidirectional 60 Hz driving deteriorates flicker characteristics so that the quality of light may be problematic. Also, in the case of the use of high-voltage AC power, there is a limitation in achieving efficient driving with the use of a single LED that commonly has a driving voltage Vf of 3V to 4V. Therefore, a high-voltage LED, permitting bidirectional operation at 120 Hz and having a high driving voltage Vf, is required to design an AC driven light emitting device.